The present invention relates to treatment of cancer, and more particularly to an apparatus and method for delivering ionizing energy to a cancerous volume of tissue from a body conduit while minimizing adverse effects on intervening healthy tissue.
Ionizing radiation treatment of cancer has a therapeutic goal of effectively treating the disease without causing intolerable injury in the process. In the context of treating cancerous body tissues by tissue destruction, such as by delivery of ionizing radiation, a more specific statement of the goal is to destroy a targeted volume of cancerous tissue without destroying healthy tissue. Where ionizing radiation is used, it is important to deliver a sufficient radiation dose to effectively destroy the targeted cancerous tissue while limiting the dose delivered to healthy tissue to a tolerable level.
Ionizing radiation is useful for treating cancer because at certain doses it has a somewhat selective injurious effect on cancerous cells as compared to healthy cells. Cancerous cells have a shorter reproductive life cycle time than normal cells, by definition. Cells are more vulnerable to damage from radiation at certain phases of the reproductive life cycle than others. For a predetermined number of cells, at any given time, more cancerous cells than normal cells are in a vulnerable phase and are therefore vulnerable to radiation damage. Consequently, in the same amount of time, more cancerous cells are injured than healthy cells by the same dose of radiation. Due to the more frequent occurrence of reproductive cell cycle phases, cancerous cells have a shorter opportunity to repair damage and therefore are more statistically likely to be irreversibly damaged by radiation than normal cells. Intermittent or fractionated doses of radiation further improve the ability of radiation therapy to destroy a greater proportion of cancerous tissue.
One technique employed to selectively treat cancerous body tissue is known as external beam radiation therapy (EBRT). According to this method, a target volume of cancerous tissue is located and an external ionizing radiation beam is sequentially focused on the target tissue from multiple angles. The intensity of ionizing radiation from a point source is generally inversely proportional to the square of the distance from the source, so that only a relatively small dose of energy may be delivered from a significant distance to the target tissue by the beam without delivering unacceptably high amounts of energy to intervening tissue between the external beam source and the target tissue. However, this effect is mitigated somewhat by employment of multiple beam angles, with the beam angles being selected to overlap only within the target volume, so that the dose delivered to the target tissue is the sum of the doses delivered by each beam while the dose delivered to intervening tissue is only that provided by a single beam. Another method utilized with EBRT involves delivering fractionated radiation doses. This technique, using multiple sublethal doses, allows intervening healthy tissue to repair the damage induced by radiation in the interval between doses, whereas a greater proportion of cancerous cells undergo reproductive cycle phases. The crossed-beam approach enables EBRT to be used with improved therapeutic effect, although the dose deliverable to the target tissue without harming intervening tissue remains less than optimal.
One significant problem with EBRT is the margin necessary to accommodate movement or shifting of the prostatic capsule. The prostate is not bound in a particular position within the pelvic region of the body, allowing the prostatic capsule to shift upon movement of the body. Mapping of the location of cancerous tissue within the prostate is typically performed in a procedure that is separate from the actual EBRT procedure, such that the cancerous tissue in the prostatic capsule may have shifted from its mapped position during the subsequent EBRT session. In order to ensure that a sufficient dose of radiation is delivered to the cancerous tissue, a margin must be provided to accommodate all of the possible positions of the cancerous tissue; that is, the volume within which the beam angles overlap is increased to include this margin. As a result, healthy tissue located within the margin is necessarily exposed to a higher dose of radiation than would otherwise be desired, limiting the dose deliverable to the cancerous tissue without causing intolerable damage to healthy tissue.
Another approach to treatment of cancerous body tissue involves combining EBRT with radioactive seed implantation, or interstitial brachytherapy. This procedure involves implanting encapsulated radioisotopes in or near the cancerous tissue to be treated, thereby delivering the highest dose of energy to tissue immediately adjacent the radiating seeds, in addition to performing crossed-beam EBRT to deliver radiation energy to the target volume. Seed therapy is often referred to by the broad term brachytherapy, meaning therapy delivered by a source located near or within the diseased area to be treated. The use of radioactive seeds allows the EBRT dose to be reduced. Consequently, the total dose to healthy tissue is reduced, resulting in reduced morbidity (i.e., impotence, incontinence, inflammation). In addition, the problems of displacement of cancerous tissue due to shifting of the prostatic capsule are reduced, since the seeds are able to move along with the targeted cancerous tissue. However, the use of radioactive seeds typically requires an invasive, interstitial implantation procedure that increases the complexity of the procedure and presents a risk of residual morbidity and side effects such as infection, incontinence, or impotence.
There is a continuing need for a minimally invasive solution to deliver effective doses of ionizing radiation to cancerous tissue while controlling doses to healthy tissue at tolerable levels.